So...? Silver and Steele and the other "humans" can turn into those..... lizard-things whenever they want? or am I missing something?

They were those lizard-things until recently.

"prototyping" is a process in which Darnathi undergo genetic modification to appear as another species. They can't transform on their own like werewolves, they need a machine which emits a special concentrated mist that changes them into another speices. Also it is effected by how old the Darnathi is when they do it, as it is very rough on their system. A younger individual (like Silver) can prototype multiple times within the span of an earth-year. (though a month between the procedure is recommended, and it's not recommended that they do this more than 2-3 times a year if they're are inclined to.) Older Darnathi (like Darius) should only prototype once past a certain age and successive attempts to Prototype can take a massive toll on the body or potentially kill them.

So no, they can't turn into their basic forms whenever they want.

The "lizard" form you see is what they are born to look like, the human form they have now is the result of them using the prototyper machine.

cool! I actually seem to be more fond of their natural form.